Method for manufacturing a substrate for use in a stamper manufacturing process, as well as a substrate obtained by using such a method

ABSTRACT

The present invention relates to a method for manufacturing a substrate for use in a stamper manufacturing process. The present invention furthermore relates to a substrate which is used in a stamper manufacturing process and to a method for manufacturing a stamper to be used in producing optical discs, comprising the exposing, developing and heating of a photosensitive film that has been coated onto a substrate.

[0001] The present invention relates to a method for manufacturing asubstrate for use in a stamper manufacturing process. The presentinvention furthermore relates to a substrate which is used in a stampermanufacturing process and to a method for manufacturing a stamper to beused in producing optical discs, comprising the exposing, developing andheating of a photosensitive film that has been coated onto a substrate.

[0002] From JP-58-077044 there is known a method for enhancing thecontact intensity of a photoresist film by producing a crosslinkingreaction after applying a thin photoresist coating to a glass substrateor to a reflective film present on the substrate. Thus a reflective filmis first formed on a glass substrate, after which a photoresist film iscoated thereon in a thickness of about 100 Å. By increasing thetemperature, a cross-linking reaction is produced between said twofilms, and a photoresist film having a thickness of about 1,000 Å iscoated onto the thus cross-linked films, followed by exposure anddevelopment thereof. The said publication does not provide anyinformation as regards the influence of the crosslinked undercoating onthe photoresist film that is finally to be exposed and developed.

[0003] Optical discs contain information that is recorded in a geometricstructure. More in particular, said geometric structure consists of pitsand lands, which pits are arranged in concentrical circles or in aspiral in a surface of a respective optical disc. Also grooves are usedas the structure of optical discs. When large numbers of such opticaldiscs are to be produced, however, a stamper is placed in the mould of,an injection moulding machine, after which the optical discs are finallymetallized and provided with a protective lacquer and a label or print.

[0004] The stamper manufacturing process as referred to in theintroduction is known per se from the prior Dutch patent applicationsnos. 9400225 and 1007216 filed in the name of the present applicant,according to which process an unstructured stamper plate is prepared,which preparation comprises the cleaning died possible coating of anadhesion layer for the photoresist that is yet to be coatedSubsequently, a negative photoresist is coated unto a thus prepared,unstructured stamper plate, which photoresist is selectively exposed tolaser light, after which the selectively exposed photoresist film isheated and subsequently integrally exposed. Finally the integrallyexposed photoresist is developed so as to provide the structuredphotoresist film, which is subsequently healed. In the direct stamperrecording (DSR) process that is currently used, in respect of which thepresent applicant was granted Dutch patent no. 1009106 before, a stamperis formed directly on a nickel substrate by means of a galvanic process.The substrate that is used thereby actually consists of a circularnickel disc having a diameter of about 200 μm and a thickness of about300 μm, onto which disc a novolak-based photoresist has been coated,using an adhesion layer. The function of said adhesion layer, which, onaccount of its thickness, can only be considered to be a “monolayer”, isto form a suitable surface onto which the novolak-based photoresist canbe coated. In the subsequent step of the DSR process, the photoresistfilm that has been coated onto the substrate is selectively exposed bymeans of laser light, and subsequently heated and integrally exposed.Following the developing step, removal of the photoresist takes place,with the exception of the photoresist posts that remain behind on thenickel substrate. Said photoresist posts are subsequently hardened byusing all exposure step with a wavelength in the deep UV region,followed by a heating step at a high temperature (hard bake) so as tostrengthen the exposed, and thus crosslinked, regions additional byfurther crosslinking of polymer chains. A stamper obtained by using sucha method thus comprises a nickel shell including photoresist posts.Further hardening of the photoresist posts makes it possible to produceat least 5000 replicas by means of such a stamper, which is used in themould of an injection moulding machine for mass-producing CD replicas.

[0005] Research has shown that defects, such as stripes and stains, maybe introduced on the surface of the nickel substrate during the galvanicprocess. In some cases said defects can be detected on an unprocessednickel substrate by means of halogen light. After the above-describedprocessing steps in the DSR process, however, such defects will show up.This phenomenon is assumed to be caused by a local difference in thediffraction pattern, which pattern is formed by the structure ofphotoresist posts. Not only the human eye is very sensitive todifferences in the diffraction pattern, but also the electric signalfrom the stamper can be influenced by the quality of the substrates.

[0006] Additional research has moreover shown that standing waves occurin the photoresist film during the exposure step of the stampermanufacturing process. Said standing waves, which are caused byreflection of the substrate, result in the occurrence of alternatinghigh and low exposure energy levels in the thickness of the resist, as aresult of which photoresist posts of irregular shape and dimensions areeventually obtained, which is undesirable.

[0007] The object of the present invention is thus to manufacture asubstrate for use in a stamper manufacturing process, in particular inthe DSR process, wherein the quality of the substrate in the stamperthat is eventually produced is of secondary importance.

[0008] Another object of the present invention is to provide a stamper,which stamper has an extended technical life span in comparison with theDSR stampers that are currently commercially available.

[0009] Another object of the present invention is to provide a methodfor manufacturing a substrate for use in a stamper manufacturingprocess, wherein it is possible to abandon the monolayer that iscurrently being used as an adhesive.

[0010] Yet another object of the invention is to provide a method formanufacturing a substrate for use in a stamper manufacturing process,which method is carried out in such a manner that the negative effect ofstanding waves, in particular the position of the interference curve, onthe formation of the photoresist posts is minimized.

[0011] The method as referred to in the introduction is according to thepresent invention characterized in that the method comprises thefollowing steps:

[0012] a) coating a crosslinkable chemical composition onto thesubstrate;

[0013] b) subjecting the undercoating applied in step a) to acrosslinking reaction; and

[0014] c) applying a top coating of a photosensitive material to theundercoating that has been crosslinked in step b), wherein the thicknessof the crosslinked undercoating of step b) is selected so that themaximum energy intensity during the exposure step of the stampermanufacturing process occurs at the desired post height of the topcoating that is to be developed.

[0015] As already said before, the light that falls onto the substrateundergoes an endless number of reflections at the interface between theresist and the surrounding atmosphere, and also at the interface betweenthe resist and the substrate. Said entering and exiting light wavesinterfere in the resist, and a so-called interference curve is formed,which interference leads to a variation in the intensity of the energythat the resist receives across the thickness of the film layer. Byselecting the thickness of said undercoating, in accordance with thepresent invention, so that the maximum energy intensity during theexposure step of the stamper manufacturing process occurs at the desiredpost height of the photoresist posts of the top coating that is to bedeveloped, the position of the interference curve is shifted so that themaximum value of the destructive interference does not occur in theregion of the desired post height. Thus a top coating is obtained fromwhich reproducible photoresist posts having a desired height and adesired shape can be obtained.

[0016] A novolak-based photoresist is preferred as said suitable,crosslinkable chemical composition. In addition to that it is alsopossible in specific embodiments to use a material which comprisesnovolak, for example pure novolak.

[0017] It is in particular preferred to carry out step b) by using aheat treatment, in particular at a temperature ranging from 150-250° C.Such a temperature range will result in three-dimensional crosslinkingof the polymer chains of the chemical composition provided in step a),which undercoating will mask any defects in the substrate.

[0018] According to a special embodiment of the present method, step b)is carried out by using an exposure step, in particular in thewavelength range of 200-320 nm.

[0019] The exposure time is in particular 5-180 seconds, whereby theexposure step is carried out In particular while rotating and/or heatingthe substrate. Since the crosslinkable chemical composition hasundergone a crosslinking reaction in step b), the thus crosslinkedcomposition is not soluble, or only to a small degree, in the solventthat is used in the photosensitive material that has been provided instep c).

[0020] According to the present method, the top coating is subjected toa heat treatment in an additional step d), which step d) is carried outat such a temperature that no crosslinking of the photosensitivematerial that has already been applied in step c) will take place. Thismeans that step d) is in particular carried out at a temperature ofmaximally 130° C. The solvent that is present in the top coating isremoved by carrying out step d).

[0021] For the present invention it is in particular important that nocrosslinking reaction will take place in the top coating of thephotosensitive material, because said top coating will be structured inthe further DSR process so as to form the above-described photoresistposts.

[0022] In a special embodiment it is desirable to subject the substrateto a preparation step prior to carrying out step a), which preparationstep comprises the cleaning of the substrate and possibly theapplication of an adhesion layer for the crosslinkable chemicalcomposition that is to be applied in step a).

[0023] The object of such a preparation step is in particular to improvethe adhesion between the substrate and the undercoating that is to beprovided in step a), whereby the undercoating is thus present on theadhesion layer that has been applied to the substrate previously.

[0024] Preferably, a metal is used for the substrate, in particularnickel.

[0025] In order to obtain layers having a uniform thickness, it ispreferred to carry out step a) as well as step c) by means of aspin-coating process, whereby the crosslinkable chemical compositionthat is coated onto the substrate in step a) may be the same as thephotosensitive material that is applied in step c).

[0026] The present invention furthermore relates to a substratethickness of the crosslinked undercoating of step b) is selected so thatthe maximum energy intensity during the exposure step of the stampermanufacturing process occurs at the desired post height of the topcoating that is to be developed. In particular the thickness of saidundercoating is in the order of 10-100 nm and wherein the thickness ofthe top coating is in the order of 150-250 nm, preferably 150-200 nm.The height of the photoresist posts is preferably 100-150 nm, inparticular 120-130 nm.

[0027] The present substrate is in particular suitable for use in amethod for manufacturing a stamper, which stamper is placed in the mouldof an injection moulding machine for mass-producing CD replicas, andsuch a method comprises the exposing, developing and heating of thephotosensitive layer which has been coated onto the substrate in stepc).

[0028] The present invention will be explained in more detail hereafterwith reference to a number of figures, which figures should not be takento be limitative, they merely function to illustrate the presentInvention.

[0029]FIG. 1 shows the shape of a photoresist post as obtained by usinga substrate not provided with an undercoating.

[0030]FIG. 2 shows the shape of another photoresist post as obtained byusing a substrate not provided with an undercoating.

[0031]FIG. 3 shows the shape of a photoresist post as obtained by usinga substrate provided with an undercoating.

[0032]FIG. 4 shows the interference curve of two embodiments, inparticular of a substrate comprising an undercoating and of a substratewhich does not comprise an undercoating.

[0033]FIG. 1 shows an example of a photoresist post wherein the standingwave effect as measured by means of AFM is clearly visible. The nickelsubstrate was provided with a photoresist having a thickness of 165 nm,the height of the photoresist posts obtained after exposure anddevelopment is 129 nm. Due to the absence of an undercoating, aphotoresist post of highly irregular shape has been obtained, which isundesirable in practice.

[0034]FIG. 2 schematically shows a photoresist post wherein the standingwave effect as measured by means of AFM is clearly visible. Present onthe nickle substrate was a photoresist film having a thickness of 165nm, with the height of the photoresist post being 121 nm. FIG. 2 clearlyshows that a photoresist post of erratic dimensions is obtained if noundercoating is used.

[0035]FIG. 3 schematically shows a photoresist post wherein a nickelsubstrate having an undercoating of 30 nm and a resist layer of 185 nmwas used. The height of the photoresist post was 122 nm. From thisschematic illustration it is apparent that the use of an undercoating ofthe desired thickness has resulted in a shift of the position of theinterference curve, which new position has led to a reproducible, smoothphotoresist post.

[0036]FIG. 4 shows the shift of the interference curve resulting fromthe use of an undercoating on a substrate. Set off on the vertical axisis parameter z, which parameter indicates the distance perpendicularlyto the substrate, wherein the value z−0 corresponds to the substratearea. Set off on the horizontal axis is the energy intensity, wherein E₀is the intensity at the resist surface, which has value 1. From FIG. 4it follows that the use of an undercoating having a thickness of 30 nmhas resulted in a shift of the position of the interference curve,wherein the maximum energy intensity occurs in particular in the regionof the desired height of the photoresist post, viz. 120-130 nm.

1. A method for manufacturing a substrate for use in a stampermanufacturing process, characterized in that said method comprises thefollowing steps: a) coating a crosslinkable chemical composition ontothe substrate; b) subjecting the undercoating applied in step a) to acrosslinking reaction; and c) applying a top coating of a photosensitivematerial to the undercoating that has been crosslinked in step b),wherein the thickness of the crosslinked undercoating of step b) isselected so that the maximum energy intensity during the exposure stepof the stamper manufacturing process occurs at the desired post heightof the top coating that is to be developed.
 2. A method according toclaim 1, characterized in that a novolak-based photoresist is used assaid suitable, crosslinkable chemical composition.
 3. A method accordingto claims 1-2, characterized in that said method furthermore comprisesan additional step d), viz.: d) subjecting the top coating applied instep c) to a heat treatment.
 4. A method according to claims 1-3,characterized in that step b) is carried out by using a heat treatment,in particular at a temperature ranging from 150-250° C.
 5. A methodaccording to claims 1-3, characterized in that step b) is carried out byusing an exposure step.
 6. A method according to claim 5, characterizedin that said exposure step is carried out in the wavelength range of200-320 nm.
 7. A method according to claims 5-6, characterized in thatthe exposure Lime is 5-180 seconds.
 8. A method according to claims 5-7,characterized in that said exposure step is carried out while rotatingthe substrate.
 9. A method according to claims 5-8, characterized inthat said exposure step is carried out while heating the substrate. 10.A method according to claims 1-9, characterized in that a layerthickness in the order of 10-100 nm is used for said undercoating instep a).
 11. A method according to claims 1-10, characterized in that alayer thickness in the order of 150-250 nm, in particular 150-200 nm, isused for the top coating in step c).
 12. A method according to claims3-11, characterized in that step d) is carried out at such a temperaturethat no crosslinking of the photosensitive material that has beenapplied in step c) takes place.
 13. A method according to claim 12,characterized in that step d) is carried out at a temperature ofmaximally 130° C.
 14. A method according to claims 1-13, characterizedin that the substrate is subjected to a preparation step prior tocarrying out step a), which preparation step comprises the cleaning ofthe substrate and possibly the application of an adhesion layer for thecrosslinkable chemical composition that is to be applied in step a). 15.A method according to claims 1-14, characterized in that a metal is usedfor the substrate, in particular nickel.
 16. A method according toclaims 1-15, characterized in that the crosslinkable chemicalcomposition that is coated onto the substrate in step a) may be the sameas the photosensitive material that is applied in step c).
 17. A methodaccording to claims 1-16, characterized in that step a) as well as stepc) are carried out by means of a spin-coating process.
 18. A substratefor use in a stamper manufacturing process, characterized in that saidsubstrate successively comprises an undercoating of a crosslinkedchemical composition and a top coating of a photosensitive material. 19.A substrate according to claim 18, characterized in that a novolak-basedphotoresist is used as said crosslinkable chemical composition.
 20. Asubstrate according to claims 18-19, characterized in that saidsubstrate is a metal, in particular nickel.
 21. A substrate according toclaims 18-20, characterized in that the thickness of said undercoatingis 10-100 nm.
 22. A substrate according to claims 18-21, characterizedin that the thickness of said top coating is 150-250 nm, in particular150-200 nm.
 23. A method for manufacturing a stamper for producingoptical discs, comprising the exposing, developing and heating of aphotosensitive layer which has been coated onto a substrate,characterized in that the substrate according to the method defined inclaims 1-17 or the substrate according to claims 18-22 is used.